difference between a ganglion and ganglion cyst ??

A ganglion is a tissue mass. A ganglion cyst is a tumor or swelling on top of a joint or the covering of a tendon (tissue that connects muscle to bone). It looks like a sac of liquid (cyst). Inside the cyst is a thick, sticky, clear, colorless, jellylike material. Depending on the size, cysts may feel firm or spongy.

What do you mean by under the arm? The arm pit? The inside of the arm at a joint? Where exactly?